Vehicle exhaust systems often include exhaust after-treatment devices that filter or otherwise treat the exhaust prior to releasing the exhaust into the environment. For example, a diesel oxidation catalyst (DOC) is a device that utilizes a chemical process in order to break down gases from a diesel engine in the exhaust flow. DOCs oxidize carbon monoxide, hydrocarbons, nitric oxide (NO), and diesel particulate matter. Additionally, DOCs may reduce nitrogen oxides (NOx) to a certain extent, although the majority of NOx reduction is achieved by a selective catalytic reduction (SCR) device that is typically placed downstream of the DOC in the exhaust flow. The SCR device converts NOx into nitrogen and water. The SCR device may be coated in a precious metal catalyst designed to trigger a chemical reaction to reduce gaseous emissions.
Exhaust from a diesel engine exhibits relatively low temperatures for a period of time following engine startup. These temperatures are typically below a minimum temperature required for an SCR device to operate with a desired efficiency in reducing NOx.